I suggest you copy the correct DuetWiFiserver-xxx.bin file to the SD card on a PC, then put the SD card back in the Duet, start the Duet, and send M997 S1 to re-flash the wifi module.

If you have flashed version 1.18.2 or earlier firmware, you need DuetWiFiServer-1.03ch.bin. if you have flashed version 1.19beta6 then you need DuetWiFiServer-1.19beta1.bin.